Liquid chromatography (LC) is a well-known technique for separating the constituent elements in a given sample. A typical liquid LC system has a sophisticated plumbing system utilizing tubing to transfer fluid between the various components of the LC system. Various fittings are used to connect the tubing to these components. Many different types of LC systems and components for LC systems are commercially available from a number of vendors. For example, Millipore Corporation of Milford, Mass., Beckman Instruments of Fullerton, California, and Hewlett-Packard Company of Palo Alto, Calif., all sell LC systems, including pumps, sample injection valves, columns, and detectors, among other things.
In a typical LC system, a liquid solvent (often called the "mobile phase") is introduced from a solvent reservoir and delivered via tubing to the pump. In operation, the pump creates a vacuum which draws the solvent through the tubing and into the pump. The solvent exits the pump under a higher pressure and then passes to the sample injection vane. As the name suggests, the sample injection vane allows an operator to inject a sample into the LC system, where the sample will be carried along with the mobile phase.
After the sample injection valve, most LC systems include a column. A typical column usually consists of a piece of steel tubing which has been packed with a "packing" material. The "packing" consists of the particulate material inside the column. This material is usually made of silica- or polymer-based particles, which are often chemically bonded with a chemical function. When the sample is carried through the column (along with the mobile phase), the various components (solutes) in the sample migrate through the packing within the column at different rates (i.e., there is differential migration of the solutes). Because of the different rates of movement, the components gradually separate as they move through the column. A more detailed description of the separation process can be found, among other places, in Chapters 2 and 5 of Introduction to Modem Liquid Chromatography (2d ed. 1979) by L. R. Snyder and J. J. Kirkland, which chapters are incorporated by reference herein.
Once the sample (with its components now separated) leaves the column, it flows with the mobile phase past a detector. The detector detects the presence of specific molecules or compounds. As discussed in Chapter 4 of Introduction to Modern Liquid Chromatography, which chapter is incorporated by reference herein, two general types of detectors are used in LC applications. One type measures a change in some overall physical property of the mobile phase and the sample (such as their refractive index). The other type measures only some property of the sample (such as the absorption of ultraviolet radiation). In essence, a typical detector in an LC system can measure and provide an output in terms of mass per unit of volume (such as grams per milliliter) or mass per unit of time (such as grams per second) of the sample's components. From such an output signal, a "chromatograph" can be provided; the chromatograph can then be used by an operator to determine the chemical components present in the sample. It will be understood to those skilled in the art that, as used herein, the term "LC system" is intended in its broad sense to include all apparatus used in connection with liquid chromatography, whether made of only a few simple components or made of numerous, sophisticated components which are computer controlled or the like.
In addition to the above components, many LC systems will include various filters, check vanes, or the like in order to prevent contamination of the sample or damage to the LC system. Often, liquid chromatography is performed with a limited amount of a given sample or an extremely expensive sample. An operator cannot afford to replicate tests at will in such situations. In addition, an operator often needs to examine extremely minute amounts of chemicals; in these situations, any contamination can potentially ruin the test. Moreover, various components of the LC system can be quite expensive. Hence, filters are used are various stages in the LC system to prevent contamination and also to protect the LC system components from wear and tear.
Most pumps used in the LC systems which are commercially available can generate relatively high pressures of up to around 10,000 to 15,000 psi. In many situations, an operator can obtain successful results by operating an LC system at low pressures of anywhere from just a few psi or so up to 1,000 psi or so. In other situations, however, an operator will find it desirable to operate an LC system at relatively "higher" pressures of over 1,000 psi. The operation and use of LC systems at such "higher" pressure levels is often referred to as "high pressure liquid chromatography" or "high performance liquid chromatography" (HPLC). In order to be suitable for HPLC applications, an LC component must be made to withstand the required pressures. Otherwise, the component may fail, thus potentially causing personal injury, the loss of valuable materials and research efforts, and the like. For these and other reasons, many components and fittings used in HPLC are made of stainless steel.
More recently, it has been realized that the use of stainless steel (and other metals) in the components of an LC system which come in contact with the mobile phase create potential drawbacks when dealing with biological samples. For example, the ions in a sample may attach themselves to the stainless steel material if the mobile phase comes in contact with the stainless steel. Similarly, ions from the metal components may detach and eventually flow past the detector, thus leading to potentially erroneous results. Hence, those portions of the LC components which come in contact with the mobile phase need to be biocompatible (i.e., chemically inert with respect to biological samples and the mobile phase carrying these samples) in many applications involving biological samples. Simply put, there is a need for biocompatible components of LC systems.
In the past, many filters for high pressure LC applications were made of metal or otherwise had metallic parts which contacted the mobile phase. For example, stainless steel and titanium filters have been used. Conventional metallic filters have been made via sintering, such as is described in U.S. Pat. No. 4,966,696, which is hereby incorporated by reference. However, such metallic filters are not biocompatible.
On the other hand, those few filters which were made of biocompatible materials (such as ultra-high molecular weight polyethylene) were made of plastic materials that could only be used with a limited number of the solvents often used as the mobile phase in many HPLC applications. For example, a "biocompatible" filter made of polyetheretherketone (PEEK) and polytetrafluoroethylene (PTFE) is commercially available from Alltech Associates, Inc. Because polytetrafluoroethylene chemically reacts with a number of mobile phases that are commonly used, however, such filters cannot be used in many LC applications. In addition, the polytetrafluoroethylene is not very strong and wears quickly. When such a filter wears down, pieces of the filter break off, thereby leading to the risk of damage of other components of the LC system and contamination of the sample to be studied. Hence, such conventional filters cannot be used in many HPLC applications. As a result, there is a need for a filter which is biocompatible and which can be used in relatively high pressure applications with a variety of mobile phases without leaking or otherwise failing.